Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 090858
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
158 AM MST Sat Dec 9 2017

.SHORT TERM...Today through Monday night

Another day of mostly clear skies and pleasant temperatures for
early December. Leaned toward persistence with temperatures today,
tonight and Sunday. Despite warm H7 temperatures, inversions in
the valleys and basins are weak (as evidenced by Friday`s highs).
Fog formation occurred after 14Z Friday in the Jackson Valley and
was confined just north of the airport. Thus, have left patchy
freezing fog in the Jackson Valley through 16Z this morning.

A cold front associated with a clipper system will pass over areas
east of the Divide early Monday morning, increasing winds once
again to the Bighorns and Johnson County. These winds should
subside through the afternoon Monday.

.LONG TERM...Tuesday through Saturday

Overview...Upper ridge over the western U.S. will continue to bring
dry and unseasonably mild weather to most areas Tuesday through
Thursday.  However, colder air will remain trapped in the Jackson
and Star Valleys under inversions, also resulting in some morning
fog.  A Pacific storm system is expected to push across the northern
Rockies in the Friday-Saturday timeframe bringing the next chances
of significant precipitation to the west and north.

Discussion...Yesterday`s high temperatures were a good indication of
where inversions may persist and not under resilient ridge across
the western U.S.  Wind River and Upper Green River Basin mixed out
for the most part, so it will be hard to get strong inversions re-
established as minimal snow-cover continues to age and melt off. The
Star and Jackson Valleys should continue to see highs mostly in the
30 to 35 degree range and lows in the single digits to near 10
Tuesday through Thursday.

A shortwave moving into the eastern Pacific over the weekend is
expected to split as it encounters strong downstream ridge over the
western U.S.  The southern piece of this split is expected to move
through/over the ridge and dive SE east of the Continental Divide
Tuesday/Wednesday.  ECMWF and Canadian models have trended slower
and further east with this system as they take system further over
the ridge rather than through it; the GFS is still hanging on to a
further southward track through the ridge.  Verification with the
last few clipper systems favor ECMWF and Canadian track SE across
the northern High Plains, so kept most areas dry Tuesday night-
Wednesday with slightly cooler highs overall on Wednesday.  The
ridge over the western U.S. is expected to retrograde into the
eastern Pacific by the end of the week with the first in a series of
Pacific shortwaves flattening the ridge, the upper air transitioning
to a more zonal flow next weekend.  Significant timing differences
remain between GFS and ECMWF with initial shortwave translating the
NW U.S. Friday-Saturday, the ECMWF about 12-18 hours faster than the
GFS.  GFS ensemble mean is a good in between solution this morning,
with higher PoPs in the west and northern mountains Friday night
and Saturday.

&&

.AVIATION...For the 12Z TAFS through 12Z Sunday

The main forecast challenge will be the coverage and duration of
fog across the far western valleys including the KJAC terminal.
Some fog formed across the western valleys Friday morning.
Atmospheric conditions (i.e., stronger surface inversion) are at
or even better for fog formation this morning, so will prevail
fog at KJAC terminal with it dissipating by midday. Elsewhere VFR
conditions with only some high-level clouds are expected.

&&

.FIRE WEATHER...

Apart from patchy freezing fog in the Jackson Valley this morning,
expect mostly clear skies, dry conditions and pleasant
temperatures (for December) over the weekend. Winds are expected
to remain light through Monday, when a cold front could cause
gusty winds over the Bighorns and Johnson County during the
morning hours Monday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...LaVoie
LONG TERM...Meunier
AVIATION...Murrell
FIRE WEATHER...LaVoie


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